How Old is the Earth

A Response to “Scientific” Creationism

he question of
the ages of the Earth and its rock formations and features has
fascinated philosophers, theologians, and scientists for
centuries, primarily because the answers put our lives in
temporal perspective. Until the 18th century, this question was
principally in the hands of theologians, who based their
calculations on biblical chronology. Bishop James Ussher, a
17th-century Irish cleric, for example, calculated that creation
occurred in 4004 B.C. There were many other such estimates, but
they invariably resulted in an Earth only a few thousand years
old.

By the late
18th century, some naturalists had begun to look closely at the
ancient rocks of the Earth. They observed that every rock
formation, no matter how ancient, appeared to be formed from
still older rocks. Comparing these rocks with the products of
present erosion, sedimentation, and earth movements, these
earliest geologists soon concluded that the time required to form
and sculpt the present Earth was immeasurably longer than had
previously been thought. James Hutton, a physician-farmer and one
of the founders of the science of geology, wrote in 1788,
“The result, therefore, of our present inquiry is, that we
find no vestige of a beginning, — no prospect of an
end.” Although this may now sound like an overstatement, it
nicely expresses the tremendous intellectual leap required when
geologic time was finally and forever severed from the artificial
limits imposed by the length of the human lifetime.

By the mid- to
late 1800s, geologists, physicists, and chemists were searching
for ways to quantify the age of the Earth. Lord Kelvin and
Clarence King calculated the length of time required for the
Earth to cool from a white-hot liquid state; they eventually
settled on 24 million years. James Joly calculated that the
Earth’s age was 89 million years on the basis of the time
required for salt to accumulate in the oceans. There were other
estimates but the calculations were hotly disputed because they
all were obviously flawed by uncertainties in both the initial
assumptions and the data.

Unbeknownst to
the scientists engaged in this controversy, however, geology was
about to be profoundly affected by the same discoveries that
revolutionized physics at the turn of the 20th century. The
discovery of radioactivity in 1896 by Henri Becquerel, the
isolation of radium by Marie Curie shortly thereafter, the
discovery of the radioactive decay laws in 1902 by Ernest
Rutherford and Frederick Soddy, the discovery of isotopes in 1910
by Soddy, and the development of the quantitative mass
spectrograph in 1914 by J. J. Thomson all formed the foundation
of modern isotopic dating methods. But it was not until the late
1950s that all the pieces were in place; by then the phenomenon
of radioactivity was understood, most of the naturally occurring
isotopes had been identified and their abundance determined,
instrumentation of the necessary sensitivity had been developed,
isotopic tracers were available in the required quantities and
purity, and the half-lives of the long-lived radioactive isotopes
were reasonably well known. By the early 1960s, most of the major
radiometric dating techniques now in use had been tested and
their general limitations were known.

No technique,
of course, is ever completely perfected and refinement continues
to this day, but for more than two decades radiometric dating
methods have been used to measure reliably the ages of rocks, the
Earth, meteorites, and, since 1969, the Moon.

Radiometric
dating is based on the decay of long-lived radioactive isotopes
that occur naturally in rocks and minerals. These parent isotopes
decay to stable daughter isotopes at rates that can be measured
experimentally and are effectively constant over time regardless
of physical or chemical conditions. There are a number of
long-lived radioactive isotopes used in radiometric dating, and
a variety of ways they are used to determine the ages of rocks,
minerals, and organic materials. Some of the isotopic parents,
end-product daughters, and half-lives involved are listed in
Table 1. Sometimes these decay schemes are used individually to
determine an age (e.g., Rb-Sr) and sometimes in combinations
(e.g., U-Th-Pb). Each of the various decay schemes and dating
methods has unique characteristics that make it applicable to
particular geologic situations. For example, a method based on a
parent isotope with a very long half-life, such as
147Sm, is not very useful for measuring the age of a rock only a few
million years old because insufficient amounts of the daughter
isotope accumulate in this short time. Likewise, the
14C method can only be used to determine the ages of certain types
of young organic material and is useless on old granites. Some
methods work only
on
closed systems, whereas others work on open
systems.1 The
point is that not all methods are applicable to all rocks of all
ages. One of the primary functions of the dating specialist
(sometimes called a geochronologist) is to select the applicable
method for the particular problem to be solved, and to design the
experiment in such a way that there will be checks on the
reliability of the results. Some of the methods have internal
checks, so that the data themselves provide good evidence of
reliability or lack thereof. Commonly, a radiometric age is
checked by other evidence, such as the relative order of rock
units as observed in the field, age measurements based on other
decay schemes, or ages on several samples from the same rock
unit. The main point is that the ages of rock formations are
rarely based on a single, isolated age measurement. On the
contrary, radiometric ages are verified whenever possible and
practical, and are evaluated by considering other relevant
data.

My
purpose here is not to review and discuss all of the dating
methods in use. Instead, I describe briefly only the three
principal methods. These are the K-Ar, Rb-Sr, and U-Pb methods.
These are the three methods most commonly used by scientists to
determine the ages of rocks because they have the broadest range
of applicability and are highly reliable when properly used.
These are also the methods most commonly criticized by creation
“scientists.” For
additional information on these methods or on methods not covered
here, the reader is referred to the books by Faul
(47), Dalrymple and Lanphere (35), Doe (38), York and Farquhar (136), Faure and Powell (50), Faure (49), and Jager and Hunziker (70), as well as the article by Dalrymple (32).

The K-Ar method
is probably the most widely used radiometric dating technique
available to geologists. It is based on the radioactivity
of 40K, which undergoes dual decay by electron capture to
40Ar and by beta emission to
40Ca. The ratio of 40K atoms that decay to
40Ar to those that decay to 40Ca is 0.117, which is called the branching ratio. Because
40Ca is practically ubiquitous in rocks and minerals and is
relatively abundant, it is usually not possible to correct for
the 40Ca initially present and so the 40K/40Ca
method is rarely used for dating. 40Ar, however, is an inert gas that escapes easily from rocks when
they are heated but is trapped within the crystal structures of
many minerals after a rock cools. Thus, in principle, while a
rock is molten the 40Ar formed by the decay of 40K escapes from the liquid. After the rock has solidified and
cooled, the radiogenic 40Ar is trapped within the solid crystals and accumulates with the
passage of time. If the rock is heated or melted at some
later time, then some or all of the 40Ar may be released and the clock partially or totally
reset.

In the process
of analysis, a correction must be made for the atmospheric
argon2
present in most
minerals and in the vacuum apparatus used for the analyses. This
correction is easily made by measuring the amount of
36Ar present and, using the known isotopic composition of
atmospheric argon (40Ar/
36Ar = 295.5),
subtracting the appropriate amount of 40Ar due to atmospheric contamination. What is left is the amount
of radiogenic 40Ar. This correction can be made very accurately and has no
appreciable effect on the calculated age unless the atmospheric
argon is a very large proportion of the total argon in the
analysis. The geochronologist takes this factor into account when
assigning experimental errors to the calculated ages.

The K-Ar method
has two principal requirements. First, there must be no argon
other than that of atmospheric composition trapped in the rock or
mineral when it forms. Second, the rock or mineral must not lose
or gain either potassium or argon from the time of its formation
to the time of analysis. By many experiments over the past three
decades, geologists have learned which types of rocks and
minerals meet these requirements and which do not. The K-Ar clock
works primarily on igneous rocks, i.e., those that form from a
rock liquid (such as lava and granite) and have simple
post-formation histories. It does not work well on sedimentary
rocks because these rocks are composed of debris from older
rocks. It does not work well on most metamorphic rocks because
this type of rock usually has a complex history, often involving
one or more heatings after initial formation. The method does
work on certain minerals that retain argon well, such as
muscovite, biotite, and volcanic feldspar, but not on other
minerals, such as feldspar from granite rocks, because they leak
their argon even at low temperatures. The method works well on
subaerial lava flows, but not on most submarine pillow basalts
because they commonly trap excess 40Ar when they solidify. One of the principal tasks of the
geochronologist is to select the type of material used for a
dating analysis. A great deal of effort goes into the sample
selection, and the choices are made before the analysis, not on
the basis of the results. Mistakes do occur but they are usually
caught by the various checks employed in the well-designed
experiment.

The Rb-Sr
method is based on the radioactivity of 87Rb, which undergoes simple beta decay to 87Sr with a half-life of 48.8 billion years. Rubidium is
a major
constituent of very few minerals, but the chemistry of rubidium
is similar to that of potassium and sodium, both of which do form
many common minerals, and so rubidium occurs as a trace element
in most rocks. Because of the very long half-life of
87Rb, Rb-Sr dating is used mostly on rocks older than about 50 to
100 million years. This method is very useful on rocks with
complex histories because the daughter product, strontium, does
not escape from minerals nearly so easily as does argon. As a
result, a sample can obey the closed-system requirements for
Rb-Sr dating over a wider range of geologic conditions than can a
sample for K-Ar dating.

Unlike argon,
which escapes easily and entirely from most molten rocks,
strontium is present as a trace element in most minerals when
they form. For this reason, simple Rb-Sr ages can be calculated
only for those minerals that are high in rubidium and contain a
negligible amount of initial strontium. In such minerals, the
calculated age is insensitive to the initial strontium amount and
composition. For most rocks, however, initial strontium is
present in significant amounts, so dating is done by the isochron
method, which completely eliminates the problem of initial
strontium.

In the Rb-Sr
isochron method, several (three or more) minerals from the same
rock, or several cogenetic rocks with different rubidium and
strontium contents, are analyzed and the data plotted on an
isochron diagram (Figure 2). The 87Rb and 87Sr
contents are normalized to the amount of 86Sr, which is not a
radiogenic daughter product. When a rock is
first formed, say from a magma, the 87Sr/86Sr ratios in
all of the minerals will be the same regardless of
the rubidium or strontium contents of the minerals, so all of the
samples will plot on a horizontal line (a-b-c in Figure 2). The
intercept of this line with the ordinate represents the isotopic
composition of the initial strontium. From then on, as each atom
of 87Rb decays to 87Sr, the points will follow the
paths3
shown by the
arrows. At any time after formation, the points will lie along
some line a’-b’-c’ (Figure 2), whose slope will
be a function of the age of the rock. The intercept of the line
on the ordinate gives the isotopic composition of the initial
strontium present when the rock formed. Note that the intercepts
of lines a-b-c and a'-b'-c' are identical, so the initial
strontium isotopic composition can be determined from this
intercept regardless of the age of the rock.

Note that the
Rb-Sr isochron method requires no knowledge or assumptions about
either the isotopic composition or the amount of the initial
daughter isotope — in fact, these are learned from the
method. The rocks or minerals must have remained systems closed
to rubidium and strontium since their formation; if this
condition is not true, then the data will not plot on an
isochron. Also, if either the initial isotopic composition of
strontium is not uniform or the samples analyzed are not
cogenetic, then the data will not fall on a straight line. As the
reader can easily see, the Rb-Sr isochron method is elegantly
self-checking. If the requirements of the method have been
violated, the data clearly show it.

An example of a
Rb-Sr isochron is shown in Figure 3, which includes analyses of
five separate phases from the meteorite Juvinas
(3). The data form an isochron indicating an age for Juvinas of
4.60 ± 0.07 billion years. This meteorite has also been
dated by the Sm-Nd isochron method, which works like the Rb-Sr
isochron method, at 4.56 ± 0.08 billion years
(84).

Figure 3:
Rb-Sr isochron for the meteorite Juvinas. The points represent
analyses on glass, tridymite and quartz, pyroxene, total rock,
and plagioclase. After Faure (49). Data from
Allegre and others (3).

The U-Pb method
relies on the decays of 235U and 238U. These two parent isotopes undergo series decay involving
several intermediate radioactive daughter isotopes before the
stable daughter product, lead (Table 1), is reached.

Two simple
independent “age” calculations can be made from the
two U-Pb decays: 238U to 206Pb, and 235U to 207Pb. In addition,
an “age” based on the 207Pb/206Pb ratio can
be calculated because this ratio changes over time.
If necessary, a correction can be made for the initial lead in
these systems using 204Pb as an index. If these three age calculations agree, then the
age represents the true age of the rock. Lead, however, is a
volatile element, and so lead loss is commonly a problem. As a
result, simple U-Pb ages are often discordant.

The U-Pb
concordia-discordia method circumvents the problem of lead loss
in discordant systems and provides an internal check on
reliability. This method involves
the 238U and 235U decays and is used in such minerals as zircon, a common
accessory mineral in igneous rocks, that contains uranium but no
or negligible initial lead. This latter requirement can be
checked, if necessary, by checking for the presence of
204Pb, which would indicate the presence and amount of initial lead.
In a closed lead-free system, a point representing the
206Pb/238U and 2O7Pb/235U ratios will plot on a curved line known as concordia (Figure 4). The location of the point on concordia depends only on the
age of the sample. If at some later date (say, 2.5 billion years
after formation) the sample loses lead in an episodic event, the
point will move off of concordia along a straight line toward the
origin. At any time after the episodic lead loss (say, 1.0
billion years later), the point Q in Figure 4 will lie on a chord
to concordia connecting the original age of the sample and the
age of the lead loss episode. This chord is called discordia. If
we now consider what would happen to several different samples,
say different zircons, from the same rock, each of which lost
differing amounts of lead during the episode, we find that at any
time after the lead loss, say today, all of the points for these
samples will lie on discordia. The upper intercept of discordia
with concordia gives the original age of the rock, or 3.5 billion
years in the example shown in Figure 4. There are several
hypotheses for the interpretation of the lower intercept, but the
most common interpretation is that it indicates the age of the
event that caused the lead loss, or 1 billion years in Figure 4.
Note that this method is not only self-checking, but it also
works on open systems. What about uranium loss? Uranium is so
refractory that its loss does not seem to be a problem. If
uranium were lost, however, the concordia-discordia plot would
indicate that also.

Figure 4:
U-Pb concordia-discordia diagram showing the evolution of a
system that is 3.5 billion years old and underwent episodic lead
loss 1.0 billion years ago. See text for explanation. After Faure
(49).

The U-Pb
concordia-discordia method is one of the most powerful and
reliable dating methods available. It is especially resistant to
heating and metamorphic events and thus is extremely useful in
rocks with complex histories. Quite often this method is used in
conjunction with the K-Ar and the Rb-Sr isochron methods to
unravel the history of metamorphic rocks, because each of these
methods responds differently to metamorphism and heating. For
example, the U-Pb discordia age might give the age of initial
formation of the rock, whereas the K-Ar method, which is
especially sensitive to argon loss by heating, might give the age
of the latest heating event.

An example of a
U-Pb discordia age is shown in Figure 5. This example shows an
age of 3.56 billion years for the oldest rocks yet found in North
America, and an age of 1.85 billion years for the latest heating
event experience by these rocks. The K-Ar ages on rocks and
minerals from this area in southwestern Minnesota also record
this 1.85-billion-year heating event.

The advocates
of “scientific” creationism frequently point to
apparent inconsistencies in radiometric dating results as
evidence invalidating the techniques. This argument is specious
and akin to concluding that all wristwatches do not work because
you happen to find one that does not keep accurate time. In fact,
the number of “wrong” ages amounts to only a few
percent of the total, and nearly all of these are due to
unrecognized geologic factors, to unintentional misapplication of
the techniques, or to technical difficulties. Like any complex
procedure, radiometric dating does not work all
the
time under all circumstances. Each technique works only under a
particular set of geologic conditions and occasionally a method
is inadvertently misapplied. In addition, scientists are
continually learning, and some of the “errors” are
not errors at all but simply results obtained in the continuing
effort to explore and improve the methods and their application.
There are, to be sure, inconsistencies, errors, and results that
are poorly understood, but
these are very few in comparison with the vast body of consistent
and sensible results that clearly indicate that the methods do
work and that the results, properly applied and carefully
evaluated, can be trusted.

Most of the
“anomalous” ages cited by creation
“scientists”
in
their attempt to discredit radiometric dating are actually
misrepresentations of the data, commonly cited out of context and
misinterpreted. A few examples will demonstrate that their
criticisms are without merit.

The creationist
author J. Woodmorappe (134) lists more than 300 supposedly “anomalous”
radiometric ages that he has culled from the scientific
literature. He claims that these examples cast serious doubt on
the validity of radiometric dating.

The
use of radiometric dating in Geology involves a very selective
acceptance of data. Discrepant dates, attributed to open systems,
may instead be evidence against the validity of radiometric
dating. (134, p. 102)

However, close
examination of his examples, a few of which are listed in Table 2, shows that he misrepresents both the data and their
meaning.

*This example was not tabulated by
Woodmorappe (134) but was discussed in his text.

Expected age(millionyears)

Age obtained(millionyears)

Formation/locality

52

39

Winona Sand/gulf coast

60

38

Not given/gulf coast

140

163,186

Coast Range batholith/Alaska

185

186-1230

Diabase dikes/Liberia

-

34,000*

Pahrump Group diabase/California

The two ages
from gulf coast localities (Table 2) are from a report by
Evernden and others (43). These are K-Ar data obtained on glauconite, a
potassium-bearing clay mineral that forms in some marine
sediment. Woodmorappe (134) fails to mention, however, that these data were obtained as
part of a controlled experiment to test, on samples of known age,
the applicability of the
K-Ar method to glauconite and to illite, another clay mineral. He
also neglects to mention that most of the 89 K-Ar ages reported
in their study agree very well with the expected ages. Evernden
and others (43) found that these clay minerals are extremely susceptible to
argon loss when heated even slightly, such as occurs when
sedimentary rocks are deeply buried. As a result, glauconite is
used for dating only with extreme caution. Woodmorappe’s
gulf coast examples are, in fact, examples from a carefully
designed experiment to test the validity of a new technique on an
untried material.

The ages from
the Coast Range batholith in Alaska (Table 2) are referenced by
Woodmorappe (134) to a report by Lanphere and others (80). Whereas Lanphere and his colleagues referred to these two K-Ar
ages of 163 and 186 million years, the ages are actually from
another report and were obtained from samples collected at two
localities in Canada, not Alaska. There is nothing wrong with
these ages; they are consistent with the known geologic relations
and represent the crystallization ages of the Canadian samples.
Where Woodmorappe obtained his 140-million-year
“expected” age is anyone’s guess because it
does not appear in the report he cites.

The Liberian
example (Table 2) is from a report by Dalrymple and others
(34). These authors studied dikes of basalt that intruded
Precambrian crystalline basement rocks and Mesozoic sedimentary
rocks in western Liberia. The dikes cutting the Precambrian
basement gave K-Ar ages ranging from 186 to 1213 million years
(Woodmorappe erroneously lists this higher age as 1230 million
years), whereas those cutting the Mesozoic sedimentary rocks gave
K-Ar ages of from 173 to 192 million years. 40Ar/39Ar
experiments4
on
samples of the dikes showed that the dikes cutting the
Precambrian basement contained excess 40Ar and that the calculated ages of the dikes do not represent
crystallization ages. The 40Ar/39Ar experiments on the dikes that intrude the Mesozoic sedimentary
rocks, however, showed that the ages on these dikes were
reliable. Woodmorappe (134) does not mention that the experiments in this study were
designed such that the anomalous results were evident, the cause
of the anomalous results was discovered, and the crystallization
ages of the Liberian dikes were unambiguously determined. The
Liberian study is, in fact, an excellent example of how
geochronologists design experiments so that the results can be
checked and verified.

The final
example listed in Table 2 is a supposed 34 billion-year Rb-Sr
isochron age on diabase of the Pahrump Group from Panamint
Valley, California, and is referenced to a book by Faure and
Powell (50). Again, Woodmorappe (134) badly misrepresents the facts. The
“isochron”
that
Woodmorappe (134) refers to is shown in Figure 6 as it appears in Faure and
Powell (50). The data do not fall on any straight line and do not,
therefore, form an isochron. The original data are from a report
by Wasserburg and others (130), who plotted the data as shown but did not draw a
34-billion-year isochron on the diagram. The
“isochrons” lines were drawn by Faure and Powell
(50) as “reference isochrons” solely for the purpose of
showing the magnitude of the scatter in the data.

Figure 6:
The Rb-Sr “isochron” from the diabase of the Pahrump
Group, interpreted by Woodmorappe (134) as giving
a radiometric age of 34 billion years. The lines are actually
“reference” isochrons, drawn by Faure and Powell
(50) to
illustrate the extreme scatter of the data. This scatter shows
clearly that the sample has been an open system and that its age
cannot be determined from these data. Radiometric ages on related
formations indicate that the Pahrump diabase is about 1.2 billion
years old. Original data from Wasserburg and others
(130).

As discussed
above, one feature of the Rb-Sr isochron diagram is that, to a
great extent, it is self-diagnostic. The scatter of the data in
Figure 6 shows clearly that the sample has been an open system
to 87Sr (and perhaps to other isotopes as well) and that no meaningful
Rb-Sr age can be calculated from these data. This conclusion was
clearly stated by both Wasserburg and others (130) and by Faure and Powell (50). The interpretation that the data represent a 34 billion-year
isochron is solely Woodmorappe’s (134) and is patently wrong.

A
series of volcanic rocks from Reunion Island in the Indian Ocean
gives K/Ar ages ranging from 100,000 to 2 million years, whereas
the 206Pb/238U and 206Pb/207Pb
ages are from 2.2 to 4.4 billion years. The factor of
discordance between ‘ages’
is
as high as 14,000 in some samples. (77, p. 201)

There are two
things wrong with this argument. First, the lead data that
Kofahl and Segraves (77) cite, which come from a report by Oversby (102), are common lead measurements done primarily to obtain
information on the genesis of the Reunion lavas and secondarily
to estimate when the parent magma the lava was derived from was
separated from primitive mantle material. These data cannot be
used to calculate the age of the lava flows and no knowledgeable
scientist would attempt to do so. Second, the U-Pb and Pb-Pb lava
“ages” cited by Kofahl and Segraves do not appear in
Oversby’s report. The K-Ar ages are the correct ages of the
Reunion lava flows, whereas the U-Pb and Pb-Pb “ages”
do not exist! We can only speculate on where Kofahl and Segraves
obtained their numbers.

Still another
study on Hawaiian basalts obtained seven “ages” of
these basalts ranging all the way from zero years to 3.34 million
years. The authors, by an obviously unorthodox application of
statistical reasoning, felt justified in recording the
“age” of these basalts as 250,000 years.
(92, p. 147)

The data Morris
(92) refers to were published by Evernden and others
(44), but include samples from different islands that formed at
different times! The age of 3.34 million years is from the Napali
Formation on the Island of Kauai and is consistent with other
ages on this formation (86, 87). The approximate age of 250,000 years was the mean of the
results from four samples from the Island of Hawaii, which is
much younger than Kauai. Contrary to Morris’ concerns,
nothing is amiss with these data, and the statistical reasoning
used by Evernden and his colleagues is perfectly rational and
orthodox.

Many of the
rocks seem to have inherited Ar40
from the magma
from which the rocks were derived. Volcanic rocks erupted into
the ocean definitely inherit Ar40
and
helium and thus when these are dated by the K40-Ar40
clock, old ages
are obtained for very recent flows. For example, lavas taken from
the ocean bottom off the island [sic] of Hawaii on a submarine
extension of the east rift zone of Kilauea volcano gave an age of
22 million years, but the actual flow happened less than 200
years ago. (117, p. 39, and similar statements in 92)

Slusher
(117) and Morris (92) advanced this argument in an attempt to show that the K-Ar
method is unreliable, but the argument is a red
herring.

Two studies
independently discovered that the glassy margins of submarine
pillow basalts, so named because lava extruded under water forms
globular shapes resembling pillows, trap 40Ar dissolved in the melt before it can escape
(36,
101). This effect is most serious in the rims of the pillows and
increases in severity with water depth. The excess
40Ar content approaches zero toward pillow interiors, which cool
more slowly and allow the 40Ar to escape, and in water depths of less than about 1000 meters
because of the lessening of hydrostatic pressure. The purpose of
these two studies was to determine, in a controlled experiment
with samples of known age, the suitability of submarine pillow
basalts for dating, because it was suspected that such samples
might be unreliable. Such studies are not unusual because each
different type of mineral and rock has to be tested carefully
before it can be used for any radiometric dating technique. In
the case of the submarine pillow basalts, the results clearly
indicated that these rocks are unsuitable for dating, and so they
are not generally used for this purpose except in special
circumstances and unless there is some independent way of
verifying the results.

On
the other hand, many lunar rocks contain such large quantities of
what is considered to be excess argon that dating by K/Ar is not
even reported. (77, p. 200)

The citation
for this statement is to a report by Turner (128). Turner,
however, made no such comment about excess argon in lunar rocks,
and there are no data in his report on which such a conclusion
could be based. The statement by Rofahl and Segraves
(77) is simply unjustifiable.

Volcanic rocks
produced by lava flows which occurred in Hawaii in the years
1800-1801 were dated by the potassium-argon method. Excess argon
produced apparent ages ranging from 160 million to 2.96 billion
years. (77, p. 200)

Similar modern
rocks formed in 1801 near Hualalai, Hawaii, were found to give
potassium-argon ages ranging from 160 million years to 3 billion
years. (92, p. 147)

Kofahl and
Segraves (77) and Morris (92) cite a study by Funkhouser and Naughton (51) on xenolithic inclusions in the 1801 flow from Hualalai Volcano
on the Island of Hawaii.

The 1801 flow
is unusual because it carries very abundant inclusions of rocks
foreign to the lava. These inclusions, called xenoliths (meaning
foreign rocks), consist primarily of olivine, a pale-green
iron-magnesium silicate mineral. They come from deep within the
mantle and were carried upward to the surface by the lava. In the
field, they look like large raisins in a pudding and even occur
in beds piled one on top of the other, glued together by the
lava. The study by Funkhouser and Naughton (51) was on the xenoliths, not on the lava. The xenoliths, which
vary in composition and range in size from single mineral grains
to rocks as big as basketballs, do, indeed, carry excess argon in
large amounts. Funkhouser and Naughton were quite careful to
point out that the apparent “ages” they measured were
not geologically meaningful. Quite simply, xenoliths are one of
the types of rocks that cannot be dated by the K-Ar technique.
Funkhouser and Naughton were able to determine that the excess
gas resides primarily in fluid bubbles in the minerals of the
xenoliths, where it cannot escape upon reaching the surface.
Studies such as the one by Funkhouser and Naughton are routinely
done to ascertain which materials are suitable for dating and
which are not, and to determine the cause of sometimes strange
results. They are part of a continuing effort to
learn.

Two extensive
K-Ar studies on historical lava flows from around the world
(31, 79) showed that excess argon is not a serious problem for dating
lava flows. The authors of these reports “dated”
numerous lava flows whose age was known from historical records.
In nearly every case, the measured K-Ar age was zero, as expected
if excess argon is uncommon. An exception is the lava from the
1801 Hualalai flow, which is so badly contaminated by the
xenoliths that it is impossible to obtain a completely
inclusion-free sample.

Creation
“scientists” commonly criticize the systematics and
methodology of radiometric dating, often implying in the process
that scientists do not arrive at their conclusions honestly. One
of the principal practitioners of this approach is Slusher
(117), whose “Critique of Radiometric Dating” abounds
with such unjustified statements. A few examples will illustrate
that the comments by Slusher (117) and other creation “scientists” are based on
ignorance of the methods and are unfounded.

There is really
no valid way of determining what the initial amounts of
Sr87
in
rocks were. There is much juggling of numbers and equations to
get results in agreement with the U-Th-Pb “clocks.”
In all these radioactive clocks, all methods are made to give
values that fit the evolutionist’s belief as to the age of
the earth and the ages of the geological events. The reason that
the various dating methods give similar ages after
“analysis” is that they are made to do so. In the
case of the initial Sr87/Sr86
ratios, these
values can be adjusted so that any age desired is obtainable.
(117, p. 40)

As discussed
above in the section on Rb-Sr dating the simplest form of Rb-Sr
dating (i.e., dating by measuring the 87Rb and 87Sr
contents in a single sample) can be done only on those samples
that are so low in initial 87Sr that the initial Sr correction is negligible. Such samples are
rare, and so nearly all modern Rb-Sr dating is done by the
isochron method. The beauty of the Rb-Sr isochron method is that
knowledge of the initial Sr isotopic composition is not necessary
— it is one of the results obtained. Contrary to
Slusher’s (117) statement, the amount of
initial 87Sr
is not needed to solve the Rb-Sr isochron age equation, only the
current 87Sr/86Sr
ratio, and the initial 87Sr/86Sr
ratio is not adjusted for any purpose.

A
second advantage of the isochron method is that it contains
internal checks on reliability. Look again at the isochron for
the meteorite Juvinas (Figure 3). The initial 87Sr/86Sr
ratio of 0.69896 was not assumed; it was a result of the isochron
analysis. The data are straightforward (albeit technically
complex) measurements that fall on a straight line, indicating
that the meteorite has obeyed the closed-system requirement. The
decay constants used in the calculations were the same as those
in use throughout the world in
1975.5 These data were
not “made” to result in an old age, as Slusher
(117) claims. The age of 4.60 ± 0.07 billion years is a result
obtained because Juvinas is genuinely an ancient
object.

There is far
too much Ar40
in
the earth for more than a small fraction of it to have been
formed by radioactive decay of K40. This is true even if the earth were really 4.5 billion years
old. In the atmosphere of the earth, Ar40
constitutes
99.6% of the total argon. This is around 100 times the amount
that would be generated by radioactive decay over the
hypothetical 4.5 billion years. Certainly this is not produced by
an influx from outer space. Thus it would seem that a large
amount of Ar40
was
present in the beginning. Since geochronologists assume that
errors due to presence of initial Ar40
are
small, their results are highly questionable.
(117, p.39)

This statement
contains several serious errors. First, there is not
more
40Ar
in the atmosphere than can be accounted for by radioactive decay
of 40K
over 4.5 billion years. An amount of 40Ar
equivalent to all the 40Ar
now in the atmosphere could be generated in 4.5 billion years if
the Earth contained only 85 ppm potassium. Current estimates of
the composition of the Earth indicate that the crust contains
about 1.9 percent potassium and the mantle contains between 100
and 400 ppm potassium. The 40Ar
content of the atmosphere is well known and is 6.6 × 1019 grams.
The estimated 40Ar
content of the crust and mantle combined is about 4 to 19 × 1019
grams (60). Thus, the Earth and the atmosphere now contain about
equal amounts of 40Ar, and the total could be generated if the
Earth contained only 170 ppm potassium and released half of
its 40Ar
to the atmosphere. Second, there have been sufficient tests to
show that during their formation in the crust, igneous and
metamorphic rocks nearly always release their
entrapped 40Ar,
thus resetting the K-Ar clock. In addition, scientists typically
design their experiments so that anomalous results, such as might
be caused by the rare case of initial 40Ar,
are readily apparent. The study of the Liberian diabase dikes,
discussed above, is a good example of this practice.

Several
creation “scientists” have attempted to discredit
Rb-Sr isochron dating by criticizing the fundamental principles
of the method. Three of these criticisms are worth examining
because they illustrate how little these creation
“scientists” understand about the fundamentals of
geochemistry in general and about isochrons in
particular.

Now
concerning the assumption that the samples had the same initial
Sr87/Sr86
ratio, some
pertinent remarks may be made. First, if it is assumed that there
is a uniform distribution of Sr87
in
the rock, then it is assumed that there is also a uniform
distribution of Rb87. But, of course, this is not assumed by the geochronologist
since there would, by conventional theory, have to be a
clustering of his points at one position on a
Sr87/Sr86
vs.
Rb87/Sr86
graph.
(117, p. 42)

There are two
serious flaws in Slusher’s (117) argument; first, the Rb-Sr isochron method does
not
require
a uniform distribution of 87Sr.
It only requires that the Sr isotopic composition, i.e.,
the 87Sr/86Sr
ratio, be constant in all phases (commonly minerals from the same
rock) at the time the rock formed (Figure 2). Even though the
various minerals will incorporate different amounts of Sr as they
cool and form, the Sr isotopic composition will be the same
because natural processes do not significantly fractionate
isotopes with so little mass difference as 87Sr
and 86Sr.
Second, Slusher (117) has confused isotopes and elements. It would be absurd to
assume that either the amount of 87Rb or the 87Rb/86Sr ratio is uniform when a rock forms. Rb and Sr are quite
different elements and are incorporated into the various minerals
in varying proportions according to the composition and structure
of the minerals. The Rb-Sr isochron method works precisely
because the Rb/Sr ratio, expressed in the isochron diagram
as 87Rb/86Sr (Figure 2), varies from mineral to mineral at formation,
whereas the Sr isotopic composition (87Sr/86Sr ratio) does not.

Dr.
Cook has pointed out that the obtaining of the isochrons is
better explained as a natural isotopic variation effect, since
similar curves are obtained for plots of Fe54/Sr86
vs
Fe58/Sr86
which are known
not to be time functions since these ratios have nothing to do
with radioactivity because these isotopes are not radioactive.
There is no way to correct for this natural isotopic variation
since there is no way to determine it. This renders the
Rb87-Sr87
series useless
as a clock. (117, p. 42)

Slusher
(117) is wrong again. He has used an invalid analogy and come to an
erroneous conclusion. 54Fe and 58Fe are naturally occurring isotopes of iron whose abundance is
5.8 and 0.3 percent, respectively, of the total iron. All a plot
of 54Fe/86Sr ratio versus 58Fe/86Sr ratio demonstrates is that (1) the Fe/Sr ratio
is not constant, and (2) the 54Fe content increases with the S8Fe content; both are expectable results. The slope of the line in
such a plot is simply the natural abundance 54Fe/58Fe ratio. The same sort of line will be obtained by plotting any
pair of naturally occurring isotopes of the same element
normalized by any nonradiogenic isotope, including
87Rb/86Sr ratio versus 85Rb/86Sr ratio. Contrary to Slusher’s (117) statement, these plots demonstrate only elemental variations in
nature, not isotopic fractionation, and they have nothing to do
with the validity of the Rb-Sr isochron.

The Rb-Sr
isochron differs from Slusher’s (117)
analogy in a very important way; i.e., the 87Sr/86Sr
ratio in a system, plotted on the ordinate (Figure 2), can
only vary by radioactive decay of 87Rb, plotted on the abscissa, over time. In comparing the Rb-Sr
isochron diagram with Cook’s Fe/Sr diagram, Slusher
(117) is merely showing that he does not understand
either.

Arndts and
Overn (8) and Kramer and others (78) claim that Rb-Sr isochrons are the result of mixing, rather
than of decay of 87Rb over long periods:

It
is clear that mixing of pre-existent materials will yield a
linear array of isotopic ratios. We need not assume that the
isotopes, assumed to be daughter isotopes, were in fact produced
in the rock by radioactive decay. Thus the assumption of immense
ages has not been proven.

The
straight lines, which seem to make radiometric dating meaningful,
are easily assumed to be the result of simple mixing.
(8, p. 6)

These authors
note that it is mathematically possible to form a straight line
on a Rb-Sr isochron diagram by mixing, in various proportions,
two end members of different 87Sr/86Sr and 87Rb/86Sr compositions.

A test
sometimes employed to check for mixing is to plot the
87Sr/86Sr ratio against 1/Sr (49). This plot shows whether the 87Sr/86Sr ratio varies systematically with the Sr content of the various
samples analyzed, as would be the case if the isochron were due
to mixing rather than radioactive to decay over time. Kramer and
others (78) have analyzed the data from 18 Rb-Sr isochrons published in the
scientific literature by plotting the 87Sr/86Sr ratio versus 1/Sr and calculating the correlation coefficient
(C.C.) to test for linear relations:

We
found that 8 (44%) had a C.C. in excess of .9; 5 additional (28%)
had a C.C. in excess of .8; 1 additional (6%) had a C.C.
in excess of .7; 2
additional (11%) had a C.C. in excess of .6; and 2 (11%) had a
C.C. less than .5 …

This
preliminary study of the recent evolutionary literature would
suggest that there are many published Rb-Sr isochrons with
allegedly measured ages of hundreds of millions of years which
easily meet the criteria for mixing, and are therefore more
cogently indicative of recent origin. (78, p.2)

Whereas a
linear plot on a diagram of 87Sr/86Sr versus 1/Sr is a necessary consequence of mixing, it is not a
sufficient test for mixing. Kramer and others
(
78) and Arndts
and Overn (8) have come to an incorrect conclusion because they have ignored
several important facts about the geochemistry of Rb-Sr systems
and the systematics of isochrons.

First, the
chemical properties of rubidium and strontium are quite
different, and thus their behavior in minerals is dissimilar.
Both are trace elements and rarely form minerals of their own.
Rubidium is an alkali metal, with a valence of +1 and an ionic
radius of 1.48 Å. It is chemically similar to potassium and
tends to substitute for that element in minerals in which
potassium is a major constituent, such as potassium feldspar and
the micas muscovite and biotite. Strontium, on the other hand, is
an alkaline-earth element, with a valence of +2 and an ionic
radius of 1.13 Å. It commonly substitutes for calcium in
calcium minerals, such as the plagioclase feldspars. The chemical
properties of rubidium and strontium are so dissimilar that
minerals which readily accept rubidium into their crystal
structure tend to exclude strontium and vice versa. Thus,
rubidium and strontium in minerals tend to be inversely
correlated; minerals high in rubidium are generally low in
strontium and vice versa. Because minerals high in rubidium will
also have higher 87Sr/86Sr ratios within a given period than those low in rubidium (see
Figure 2), the 87Sr/86S
r ratio commonly is inversely correlated with the Sr content.
Thus, mineral and rock isochron data will commonly show a
quasi-linear relation on a diagram of 87Sr/86Sr versus 1/Sr, with
the 87Sr/86Sr ratio increasing with increasing 1/Sr. This relation, however,
is a natural consequence of the chemical behavior of rubidium and
strontium in minerals and of the decay of 87Rb to 87Sr over time, and has nothing to do with mixing.

Second, mixing
is a mechanical process that is physically possible only in those
rock systems where two or more components with different chemical
and isotopic compositions are available for mixing. Examples
include the mingling of waters from two streams, the mixing of
sediment from two different source rocks, and the contamination
of lava from the mantle by interactions with the crustal rocks
through which it travels to the surface. Mixing in such systems
has been found (49, 70), but the Rb-Sr method is rarely used on these systems. The
Rb-Sr isochron method is most commonly used on igneous
rocks, which form by cooling from a liquid. Mineral composition
and the sequence of mineral formation are governed by chemical
laws and do not involve mixing. In addition, a rock melt does not
contain isotopic end members that can be mechanically mixed in
different proportions into the various minerals as they form, nor
could such end members be preserved if they were injected into a
melt.

Third, how
could an end member with a high 87Sr/86Sr ratio exist if this ratio ultimately were not due to the decay
of 87Rb over time? Even if isochrons were the result of mixing —
which they are not — the existence of a high 87Sr/86Sr ratio end
member would indicate the passage of billions of
years.

Fourth, if
isochrons were the result of mixing, approximately half of them
should have negative slopes. In fact, negative slopes are
exceedingly rare and are confined to those types of systems,
mentioned above, in which mechanical mixing is possible and
evident.

Finally, there
are numerous isochrons that do not show a positive correlation on
a diagram of the 87Sr/86Sr versus 1/Sr. An example is the meteorite Juvinas (Figure 3). A
plot of the 87Sr/86Sr ratio versus 1/Sr for this meteorite (Figure 7) shows clearly
that there is no linear relation. Thus, even using the criteria
developed by Arndts and Overn (8) and Kramer and others (78), the 4.6-billion-year isochron for Juvinas must be accepted as
representing a valid crystallization age.

Figure 7:
87Sr/86Sr ratio versus 1/Sr for the meteorite Juvinas. The
absence of a linear relation proves that the isochron shown in
Figure 3 could not be due to mixing. Data from Allegre and others
(3).

Therefore,
arguments advance by Arndts and Overn (8) and by Kramer and others (78) are based on premises that are geochemically and logically
unsound, and their conclusion that isochrons are due to mixing
rather than to decay of 87Rb over geologic time is incorrect.

The
radioactivity of carbon-14 is very weak and even with all its
dubious assumptions the method is not applicable to samples that
supposedly go back 10,000 to 15,000 years. In those intervals of
time the radioactivity from the carbon-14 would become so weak
that it could not be measured with the best of instruments.
Claims have been made that dating can be done back to from 40 to
70 thousand years, but it seems highly improbable that
instruments could measure activity of the small amounts of
C14
that would be
present in a sample more than 15,000 years old.
(117, p. 45)

This statement
was as untrue when it was first written in 1973
(117, 1973 ed., p. 35) as it is today. Modern counting instruments,
available for more than two decades, are capable of counting
the 14C activity in a sample as old as 35,000 years in an ordinary
laboratory, and as old as 50,000 years in laboratories
constructed with special shielding against cosmic radiation. New
techniques using accelerators and highly sensitive mass
spectrometers, now in the experimental stage, have pushed these
limits back to 70,000 or 80,000 years and may extend them beyond
100,000 years in the near future.

Creation
“scientists” commonly claim that the process of
radioactive decay is not constant. Before discussing some of
their claims, it is worth discussing briefly the types of
radioactive decay and the evidence that decay is constant over
the range of conditions undergone by the rocks available to
scientists.

Most
radioactive decay involves the ejection of one or more
sub-atomic particles from the nucleus. Alpha decay occurs when an
alpha particle (a helium nucleus), consisting of two protons and
two neutrons, is ejected from the nucleus of the parent isotope.
Beta decay involves the ejection of a beta particle (an electron)
from the nucleus. Gamma rays (very small bundles of energy) are
the device by which an atom rids itself of excess energy. Because
these types of radioactive decay occur spontaneously in the
nucleus of an atom, the decay rates are essentially unaffected by
physical or chemical conditions. The reasons for this are that
nuclear forces act over distances much smaller than the distances
between nuclei, and that the amounts of energy involved in
nuclear transformations are much greater than those involved in
normal chemical reactions or normal physical conditions. Putting
it another way, the “glue” holding the nucleus
together is extremely effective, and the nucleus is well
insulated from the external world by the electron cloud
surrounding every atom. This combination of the strength of
nuclear binding and the insulation of the nucleus is the reason
why scientists must use powerful accelerators or atomic reactors
to penetrate and induce changes in the nuclei of
atoms.

A great many
experiments have been done in attempts to change radioactive
decay rates, but these experiments have invariably failed to
produce any significant changes. It has been found, for example,
that decay constants are the same at a temperature of 2000°C
or at a temperature of -186°C and are the same in a vacuum or
under a pressure of several thousand atmospheres. Measurements of
decay rates under differing gravitational and magnetic fields
also have yielded negative results. Although changes in alpha and
beta decay rates are theoretically possible, theory also
predicts that such changes would be very small
(42) and thus would not affect dating methods.
Under certain environmental conditions, the decay characteristics
of 14C, 60Co, and 137Ce,
all of which decay by beta emission, do deviate slightly from
the ideal random distribution predicted by current theory
(5,
6), but changes
in the decay constants have not been detected.

There is a
fourth type of decay that can be affected by physical and
chemical conditions, though only very slightly. This type of
decay is electron capture (e.c. or K-capture), in which an
orbital electron is captured by the nucleus and a proton is
converted into a neutron. Because this type of decay involves a
particle outside the nucleus, the decay rate may be affected by
variations in the electron density near the nucleus of the atom.
For example, the decay constant of 7Be in different beryllium chemical compounds varies by as much as
0.18 percent (42, 64,). The only isotope of geologic interest that undergoes e.c.
decay is 40K, which is the parent isotope in the K-Ar method. Measurements
of the decay rate of 40K in different substances under various conditions indicate that
variations in the chemical and physical environment have no
detectable effect on its e.c. decay constant.

Another type of
decay for which small changes in rate have been observed is
internal conversion (IC). During i nternal conversion, however,
an atom’s nucleus goes from one energy state to a lower
energy state; it does not involve any elemental transmutation and
is, therefore, of little relevance to radiometric dating
methods.

Slusher
(115, p. 283) states that “there is excellent laboratory
evidence that external influences can change the decay
rates,” but the examples he cites are either IC or e.c.
decays with exceedingly small changes in rates. For example, in
the first (1973) edition of his monograph on radiometric dating,
Slusher (117) claims that the decay rate of 57Fe has been changed by as much as 3 percent by electric fields;
however this is an IC decay, and
57Fe remains Fe. Note, however, that even a 3 percent change in the
decay constants of our radiometric clocks would still leave us
with the inescapable conclusion that the Earth is more than 4
billion years old. DeYoung (37) lists 20 isotopes whose decay rates have been changed by
environmental conditions, alluding to the possible significance
of these changes to geochronology, but the only significant
changes are for isotopes that “decay” by internal
conversion. These changes are irrelevant to radiometric dating
methods.

Morris
(92) claims that free neutrons might change decay rates, but his
arguments show that he does not understand either neutron
reactions or radioactive decay. Neutron reactions do not change
decay rates but, instead, transmute one nuclide into another. The
result of the reaction depends on the properties of the target
isotope and on the energy of the penetrating neutron. There are
no neutron reactions that produce the
same result as
either beta or alpha decay. An (n,p) (neutron in, proton out)
reaction produces the same change in the nucleus of an atom as
e.c. decay, but there are simply not enough free neutrons in
nature to affect any of the isotopes used in radiometric dating.
If enough free neutrons did exist, they would produce other
measurable nuclear transformations in common elements that would
clearly indicate the occurrence of such a process. No such
transformations have been found, and so Morris’ claims are
disproved.

Morris
(92) also suggests that neutrinos might change decay rates, citing a
column by Jueneman (72) in Industrial Research.
The subtitle of Jueneman’s
columns, which appear regularly, is, appropriately,
“Scientific Speculation.” He speculates that
neutrinos released in a supernova explosion might have
“re-set” all the radiometric clocks. Jueneman
describes a highly speculative hypothesis that would account for
radioactive decay by interaction with neutrinos rather than by
spontaneous decay, and he notes that an event that temporarily
increased the neutrino flux might “reset” the clocks.
Jueneman, however, does not propose that decay rates would be
changed, nor does he state how the clocks would be reset; in
addition, there is no evidence to support his speculation.
Neutrinos are particles that are emitted during beta decay. They
have no charge and very small or possibly no rest mass. Their
existence was proposed by Wolfgang Pauli in 1931 to explain why
beta particles are given off with a wide range of energies from
any one isotope, rather than with a constant energy; the
“missing” energy is carried off by the neutrino.
Because they have no charge and little or no mass, neutrinos do
not interact much with matter — most pass unimpeded right
through the Earth — and they can be detected experimentally
only with great difficulty. The chance that neutrinos could have
any effect on decay rates or produce nuclear transmutations in
sufficient amounts to have any significant effect on our
radiometric clocks is exceedingly small.

Slusher
(117) and Rybka (110) also propose that neutrinos can change decay rates, citing an
hypothesis by Dudley (40) that decay is triggered by neutrinos in a “
neutrino sea” and that changes in the neutrino flux might
affect decay rates. This argument has been refuted by Brush
(20), who points out that Dudley’s hypothesis not only
requires rejection of both relativity and quantum mechanics, two
of the most spectacularly successful theories in modern science,
but is disproved by recent experiments. Dudley himself rejects
the conclusions drawn from his hypothesis by Slusher
(117) and Rybka (110), noting that the observed changes in decay rates are
insufficient to change the age of the Earth by more than a few
percent (Dudley, personal communication, 1981, quoted in
20, p. 51). Thus, even if Slusher and Rybka were correct —
which they are not — the measured age of the Earth would
still exceed 4 billion years.

Slusher
(115, 117) and Rybka (110) also claim that the evidence from pleochroic
halos6
indicates that
decay rates have not been constant over time:

…
evolutionist geologists have long ignored the evidence of
variability in the radii of pleochroic halos, which shows that
the decay rates are not constant and would, thus, deny that some
radioactive elements such as uranium could be clocks.
(115, p. 283)

In
a review of the subject, however, Gentry (52)
concludes that the data from pleochroic halo studies are
inconclusive on this point — the uncertainties in the
measurements and other factors are too great.

Two
cases where it appears that the half life is increasing with time
are as follows. Glasstone (1950) has the half life for Protactinium 231 as 3.2 × 104
years while
Kaplan (1962) has the half life as 3.43 × 104
years. For the
half life of Radium 223, Glasstone has 11.2 days while Kaplan has
11.68 days. (110, p. ii)

Rybka’s
(110) analysis of the situation, however, is wrong. He has failed to
consider all of the data.

The various
values for the half lives of 223Ra and 231Pa
reported in the literature since 1918 are given in Table 3. It
is clear that there is no increase in the values as a function of
time. The differences in the reported half lives are a
consequence of improved methods and instruments, and the care
with which the individual measurements were made. For example,
Kirby and others (74) argue convincingly that the measurements of the half life
of 223Ra reported from 1953 to 1959 (Table 3) suffered from inadequate
experimental methods and are not definitive. Kirby and his
colleagues carefully measured this half life by two different
methods and obtained values of 11.4347 ± 0.0011 days and
11.4267 ± 0.0062 days. The weighted mean of these two
measurements is 11.4346 ± 0.0011 days, which currently is
the best value for the half life of 223Ra. I should also mention that the two references cited by Rybka
are textbooks, not the publications in which the original data
were reported; the dates of publication of these texts,
therefore, do not reflect the years in which the measurements
were made or reported.

Table 3: Measurements of the Half-lives of 223Ra and 231Pa. Data
from Lederer and Shirley (81), Kirby et
al. (74), and
references therein

Nuclide

Year Reported

Half-Life

223Ra

1918

11.2 days

1953

11.1 days

1954

11.685 days

1959

11.22 days

1959

11.41 days

1965

11.4346 days

231Pa

1930

3.2 × 104 years

1932

3.2 × 104 years

1949

3.43 × 104 years

1968

3.234 × 104 years

1969

3.276 × 104 years

1977

3.276 × 104 years

Rybka
(110) also explores the consequences of a hypothetical change over
time of the decay constant, but his results are due solely to his
arbitrary changes in the decay formula — changes for which
there is neither a theoretical basis nor a shred of physical
evidence.

In summary, the
attempts by creation “scientists” to attack the
reliability of radiometric dating by invoking changes in decay
rates are meritless. There have been no changes observed in the
decay constants of those isotopes used for dating, and the
changes induced in the decay rates of other radioactive isotopes
are negligible. These observations are consistent with theory,
which predicts that such changes should be very small. Any
inaccuracies in radiometric dating due to changes in decay rates
can amount to, at most, a few percent.

Several
creationist authors have criticized the reliability of
radiometric dating by claiming that some of the decay constants,
particularly those for 40K, are not well known (28, 29, 92, 117). A common assertion is that these constants are
“juggled” to bring results into agreement; for
example:

The
so-called “branching ratio”, which determines the
amount of the decay product that becomes argon (instead of
calcium) is unknown by a factor of up to 50 percent. Since the
decay rate is also unsettled, values of these constants are
chosen which bring potassium dates into as close correlation with
uranium dates as possible. (92, p. 145)

There seems to
be some difficulty in determining the decay constants for the
K40-Ar40
system.
Geochronologists use the branching ratio as a semi-empirical,
adjustable constant which they manipulate instead of using an
accurate half-life for K40. (117, p. 40)

These
statements would have been true in the 1940s and early 1950s,
when the K-Ar method was first being tested, but they were not
true when Morris (92) and Slusher (117) wrote them. By the mid- to late 1950s the decay constants and
branching ratio of 40K were known to within a few percent from direct laboratory
counting experiments (2). Today, all the constants for the isotopes used in radiometric
dating are known to better than 1 percent. Morris
(92) and Slusher (117) have selected obsolete information out of old literature and
tried to represent it as the current state of
knowledge.

In spite of the
claims by Cook (28, 29),
Morris (92), Slusher (115, 117),
DeYoung (37) and Rybka (110),
neither decay rates nor abundance constants are a significant
source of error in any of the principal radiometric dating
methods. The reader can easily satisfy himself on this point by
reading the report by Steiger and Jaeger (124)
and the references cited therein.

Neutron
reaction corrections in the U-Th-Pb series reduce
“ages”
of
billions of years to a few thousand years because most of the Pb
can be attributed to neutron reactions rather than to radioactive
decay. (117, p. 54)

Statements
similar to this one by Slusher (117)
are also made by Morris (92). These statements spring from an argument developed by Cook
(28) that involves the use of incorrect assumptions and nonexistent
data.

Cook’s
(28) argument, repeated in some detail by Morris
(92) and Slusher
(117), is based on U and Pb isotopic measurements made in the late
1930s and early 1950s on uranium ore samples from Shinkolobwe,
Katanga and Martin Lake, Canada. Here, I use the Katanga example
to show the fatal errors in Cook’s (28) proposition.

Table 4: Uranium, Thorium, and Lead Analyses on a Sample (Nier 2)
of Uranium Ore from Shinkolobwe, Katanga, as Reported by Faul
(46). Data from
Nier (100)

206Pb/238U age = 616 million years

206Pb/207Pb age = 610 million years

Element(weight percent in ore)

Pb isotopes(percent of total Pb)

U = 74.9

204Pb = -----

Pb = 6.7

206Pb = 94.25

Th = ---

207Pb = 5.70

208Pb = 0.042

In the late
1930s, Nier (100) published Pb isotopic analyses on 21 samples of uranium ore
from 14 localities in Africa, Europe, India, and North America
and calculated simple U-Pb ages for these samples. Some of these
data were later compiled in the book by Faul (46) that Cook (28) cites as the source of his data. Table 4 lists the data for one
typical sample. Cook notes the apparent absence of thorium
and 204Pb, and the presence of 208Pb. He reasons that the 208Pb could not have come from the decay of 232Th because thorium is absent, and could not be common lead
because 204Pb, which is present in all common lead, is absent. He reasons
that the 208Pb in these samples could only have originated by neutron
reactions with 207Pb and that 207Pb, therefore, would also be created from Pb-206 by similar
reactions:

Cook
(28) then proposes that these effects require corrections to the
measured lead isotopic ratios as follows: (1) the
206Pb lost by conve rsion to 207Pb must be added back to
the 206Pb; (2) the 207Pb lost by conversion to
208Pb must be added back to the 207Pb; and
(3) the 207Pb gained by conversion from 206Pb must be
subtracted from the 207Pb. He presents an equation for making these
corrections:

based on the
assumption that the neutron-capture cross
sections7
for
206Pb and 207Pb are equal, an assumption that Cook
(28) calls “reasonable.”
Cook then
substitutes the average values (which differ slightly from the
values listed in Table 4)
for
the Katanga analyses into his equation and calculates a corrected
ratio8:

This
calculation is repeated by both Morris (92) and
Slusher (117). Cook (28), Morris (92), and Slusher (117) all note that this ratio is close to the present day production
ratio of 206Pb and 207Pb from 238U and
235U, respectively, and conclude, therefore, that the Katanga ores
are very young, not old. For example, Slusher
(117)
states:

This corrected
ratio says the corrected age should be practically zero since
Pb206/Pb207
=
21.5 for modern radiogenic lead. (117, p. 36)

Although
Cook’s (28) logic may, superficially, seem reasonable and straightforward,
it suffers from several serious fundamental flaws. First,
204Pb is not absent in the Katanga samples; it simply was not
measured! In his report, Nier (100) states:

Actually, in 20
of the 21 samples investigated the amount of common lead is so
small that one need not take account of the variations in its
composition. In a number of samples where the abundance of
204Pb was very low no attempt was made to measure the amount of it
as the determination would be of no particular value.
(100, p. 156)

Second, the
neutron-capture cross sections for 206Pb and 207Pb are not equal, as Cook
(28) assumes, but differ by a factor of 24 (0.03 barns for
206Pb, 0.72 barns for 207Pb‡).
This discrepancy has a significant effect on the results of
Cook’s (28) calculation. Table 5 compares the results of the three methods
of age calculation — the correct method, Cook ’s
(28) method, and Cook’s method with the correct nuclear cross
sections — using the currently accepted best values for the
uranium decay rate and abundance constants. The correct
radiometric age is, of course, the scientific value of 622
million years. When Cook’s (28) calculation is done with appropriate
allowance for the unequal
neutron-capture cross sections of 206Pb and 207Pb, the resulting
calculated age is actually older
than
the scientific value, so even if such neutron reactions had
occurred, the effect would be the opposite of that claimed by
Cook (28). Note also that even Cook’s (28) incorrect calculation results in an age of 70 million years,
not “practically zero” as asserted by Slusher
(117).

Table 5: Comparison of 206Pb/207Pb Age Calculations for the
Katanga Uranium Ores, Using the Average Values from Cook
(28) and the
Modern Decay Rates and Abundance Constants

The third
problem with Cook’s proposition is that there are far too
few free neutrons available in nature, even in uranium ores, to
cause significant effects. This fact is readily acknowledged by
Cook:

In
spite of evidence that the neutron flux is only a millionth as
large as
it should be to account for appreciable (n, ) effects,
there are
several well documented examples that seem to demonstrate the
reality of this scheme. (28, p. 54)

The examples
are, of course, those from Katanga and Martin Lake.

Thus
Cook’s (28) proposition and calculations, enthusiastically endorsed by
Morris (92) and Slusher (117), are based on data that do not exist and are, in addition,
fatally flawed by demonstrably false assumptions.

1 An
isolated system is
one
in which neither matter nor energy enters or leaves. A closed
system is one in which only matter neither enters nor leaves. A
system that is not closed is an open system. A
“system” may be of any size, including very small
(like a mineral grain) or very large (like the entire universe).
For radiometric dating the system, usually a rock or some
specific mineral grains, need only be closed to the parent and
daughter isotopes.

2 Approximately
one percent of the Earth’s atmosphere is argon, of which
99.6 percent is 40Ar.

3 These
paths will be at an angle of 45° if the scales on the
abscissa and ordinate are the same.

4 The
40Ar/39Ar
technique is an analytical variation of K-Ar dating. The validity
of ages obtained by this technique can be verified from the data
alone in a manner analogous to the Rb-Sr isochron method
discussed above. For more information on 40Ar/39Ar
dating, see Dalrymple (32).

6 Pleochroic
halos are rings of discolored areas around radioactive inclusions
in some minerals. The discoloration is caused by radiation damage
to the crystals by subatomic particles. The radii of these rings
are proportional to the energies of the particles.

7 A
nuclear reaction cross section, expressed in units of area
(barns), is simply a measurement of the probability that the
particle in question will penetrate the nucleus of the target
isotope and cause the reaction in question.

8 The
values and equation actually give a result of 21.3. Cook
published a result of 21.1. I have used Cook’s result for
consistency.